Influences of seawater adaptation on the endocrine, osmoregulatory and immune systems in gill, peripheral blood leucocyte, skin and intestine of steelhead trout <scp><i>Oncorhynchus mykiss</i></scp>

Endocrine, osmoregulatory and immune responses have been examined in blood plasma, gill, peripheral blood leucocyte (PBL), skin and intestine of steelhead trout Oncorhynchus mykiss reared for 4 weeks in seawater. Plasma lysozyme activity and pituitary growth hormone messenger ribonuculeic acid (mRNA) levels were higher and prolactin levels were lower in seawater than in freshwater. mRNA levels of hormone receptor and immune-related genes in gills, PBL and skin were influenced by seawater. Especially in the intestine, mRNA levels of all examined hormone receptors were significantly higher in seawater, suggesting endocrine regulation of osmolality and immunity.